Many, MANY people have this problem with their Droid. While the phone's screen is supposed to turn off when held to the cheek....sometimes it doesn't do so.

You may have a defective sensor on your phone and that is why it's not registering when you press the speaker to your face.

To test this, try holding your finger on the sensor (not sure quite where it is on that model) and then pulling the finger away....the sensor should turn the phone on and off. If it doesn't you may have to bring the phone in and ask for a replacement. ;(

no its not the sensor thats defective. i have the same problem but learned a way around it. it seems that the sensor is not calebrated properly but you cant do anything about it. what i do is, when your display turns on,( when its supposr to be off) move the the phone a bit higher or lower on your ear. that works , or just move the phone away from face and back to ear while the problem happens. i keep my phone a bit forward so the sensor is completly covred with my ear, ever since, no cheek contact problems